R语言中向NextMethod()传入generic参数的作用与目的是什么？

示例 #

# 示例数据
x <- as.Date(c("2022-01-01", "2023-01-01", "1900-01-01", "2024-01-01")) 

# 按预期返回累积最大值
cummax.Date <- function(x, ...) .Date(NextMethod(), cl = oldClass(x))
cummax(x)
#> [1] "2022-01-01" "2023-01-01" "2023-01-01" "2024-01-01"

# 按预期返回累积最小值
cummin.Date <- function(x, ...) .Date(NextMethod(), cl = oldClass(x))
cummin(x)
#> [1] "2022-01-01" "2022-01-01" "1900-01-01" "1900-01-01"

最佳实践 #

在实际开发的最佳实践中，建议显式指定泛型函数：

cummin.Date <- function(x, ...) .Date(NextMethod(generic = "cummin"), cl = oldClass(x))
cummin(x)
#> [1] "2022-01-01" "2022-01-01" "1900-01-01" "1900-01-01"

不良实践——无差异 #

不过需要注意，显式指定的泛型函数无论是否真实存在，都不会影响最终执行结果：

# 仍然返回累积最小值
cummin.Date <- function(x, ...) .Date(NextMethod(generic = "cummax"), cl = oldClass(x))
cummin(x)
#> [1] "2022-01-01" "2022-01-01" "1900-01-01" "1900-01-01"

# 仍然返回累积最小值
cummin.Date <- function(x, ...) .Date(NextMethod(generic = "foobar"), cl = oldClass(x))
cummin(x)
#> [1] "2022-01-01" "2022-01-01" "1900-01-01" "1900-01-01"

内容的提问来源于stack exchange，提问作者mnist